1. Field of the Invention
The present invention relates to a method for producing an internally reinforced thermoplastic film web possessing an aeolotropic through coextrusion of two polymeric materials, and more particularly, a method of forming a film web possessing an enhanced tensile strength in at least the longitudinal direction of the film web. Moreover, the invention also relates to a plastic film having internal striplike reinforcements entirely embedded within a matrix material which is formed by the method pursuant to the invention, and especially pertains to articles formed from such a film; for instance, plastic bags, bag components such as bag handles or liners, stretch film, and the like.
Frequently, the utilization of articles formed of thermoplastic films which are constituted of polymers, such as polyethylene or the like, requires the film to possess a higher tensile strength in at least one direction of the film. For instance, plastic films have found widespread use in the manufacture of plastic bags or components thereof, such as supermarket grocery bags, which, when filled with product are subjected to extensive tensile stresses, and depending upon the bag construction, usually in the longitudinal direction of the film as a result of the weight of the bag contents. Such thermoplastic films are also frequently in the form of stretch films utilized as high-strength wrappers which are subjected to high tensile stresses in at least one direction of the film, usually the machine direction of the film web. Consequently, it has become desirable to reinforce such bags which are made from plastic film webs, whereby the film possesses a higher tensile strength in at least one direction of the film web; for instance, in the machine direction of the film webs.
2. Discussion of the Prior Art
Heretofore, greater tensile strength has been imparted to the film by either forming thickened portions in the thermoplastic film web during the forming thereof by extrusion through suitable slot dies or tubular dies, in which the film is imparted a somewhat higher tensile strength in the longitudinal or mechanic direction of the extruded film web.
Numerous solutions have been proposed to the problem in imparting an increased tensile strength to a thermoplastic film web, in at least one direction of the film, by the application of reinforcements to either one or both surfaces of the film web, and through adhesive or thermal bonding therewith. However, this has evidenced itself to necessitate the use of relatively complex and expensive forming processes, which will not in all instances lead to the desired increase in tensile strength or tear resistance in one direction of the thermoplastic film web, such as the machine direction of the web.
Fair, et al. U.S. Pat. No. 4,410,587 discloses a coextruded fusible thermoplastic film material in the shape of a net in which laminations are provided in order to form a center layer of a high-melt-strength polymer with a low-melt-strength polymer being applied on both sides thereof. This disclosure provides for a relatively complex and expensive mode of producing a reinforced thermoplastic film material.
Komoda, et al. U.S. Pat. No. 4,410,602 discloses an extended reclaimed synthetic resin film in which a different type of an encompassing resin layer is formed by being coextruded through a composite die. This process and the film produced thereby would not be economical for high volume thermoplastic film usage generally employed in single, throw-away applications, such as grocery sacks or the like.
Hessenthaler, et al. U.S. Pat. No. 3,034,941 discloses the formation of extruded beads or strips being applied to the surface of or incorporated into a thermoplastic film material subsequent to extrusion. This will not permit the desired additional tensile strength to be imparted to the lower strength polyethylene film in a manner as contemplated herein, and concurrently necessitates the employment of more complex and expensive equipment to form the composite plastic film wrapping material structure.
Donald U.S. Pat. No. 3,397,428 discloses the formation of composite articles formed from a thermoplastic resinous material, in which a resinous material of one type is encased within a second resinous or polyethylene material to provide an insert of higher strength. Again, this does not provide the inexpensive manner of forming an internally reinforced atropic plastic film material, particularly when desired for high-volume disposable, low-cost or grocery sacks.
Benoit, et al. U.S. Pat. No. 4,464,157 discloses a method of forming a gussetted thermoplastic bag incorporating reinforced carrying handles formed integrally with the bag walls. This type of bag construction necessitates the extrusion of thermoplastic film webs having a somewhat greater film thickness in the regions employed for the bag handles which are formed by at least two coextruded film layers. Although providing a superior thermoplastic bag with an improved tear resistance and tensile strength in at least one film direction, accurate control over the formation of the bags is rendered difficult due to the varying film thickness across the film web as the web is advanced through bag making equipment currently available in this technology.
Various other types of apparatus and methods for producing composite and/or reinforced thermoplastic film materials are described, for instance, in Corbett, et al. U.S. Pat. No. 3,372,920, and in numerous foreign publications, such as European Pat. No. 32629, and Japanese Patent Publications No. 6194, 14022, and 120792. None of these, however, describe a forming method and internally reinforced film having the aeolotropic properties contemplated herein which is particularly adapted for low-cost bags, stretch films, or similar disposable articles, and wherein these articles are formed through a coextrusion method forming an aeolotropic film analogous to that contemplated herein.